Boiling-Point Elevation and Freezing-Point Depression Constants
Molal boiling-point-elevation (Kb) and freezing-point-depression (Kf) constants for common solvents. Use ΔTb = i·Kb·m and ΔTf = i·Kf·m, where m is the molality of the solution and i is the van 't Hoff factor.
Molal Kb and Kf Constants
Apply ΔT = i·K·m for boiling-point elevation (Kb) and freezing-point depression (Kf).
| Water | H2O | 0.512 °C·kg/mol | 1.86 °C·kg/mol |
| Acetic acid | CH3COOH | 3.07 °C·kg/mol | 3.9 °C·kg/mol |
| Benzene | C6H6 | 2.53 °C·kg/mol | 5.12 °C·kg/mol |
| Chloroform | CHCl3 | 3.63 °C·kg/mol | 4.68 °C·kg/mol |
| Nitrobenzene | C6H5NO2 | 5.24 °C·kg/mol | 8.1 °C·kg/mol |
| Ethanol | C2H5OH | 1.20 °C·kg/mol | — |
| Carbon disulfide | CS2 | 2.34 °C·kg/mol | — |
| Camphor | C10H16O | — | 37.7 °C·kg/mol |
Important Notes
- ΔTb = i·Kb·m raises the boiling point; ΔTf = i·Kf·m lowers the freezing point. Both use MOLALITY (mol solute per kg solvent), not molarity.
- The van 't Hoff factor i counts dissolved particles: i = 1 for nonelectrolytes, i ≈ the ion count for strong electrolytes (e.g. i ≈ 2 for NaCl, i ≈ 3 for CaCl₂).
- A dash means that constant is not commonly tabulated for that solvent.
- Camphor's very large Kf made it the classic solvent for molar-mass determination by freezing-point depression (the Rast method).
- ChemWhiz uses this one consistent value set across the site.
Sources:
- OpenStax Chemistry 2e, Section 11.4, Table 11.2, water, acetic acid, benzene, chloroform, and nitrobenzene (CC BY 4.0)
- ChemWhiz course constants, ethanol, carbon disulfide, and camphor